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So the short answer is there's nothing in Feedburner's robots.txt to prevent crawlers from accessing those URLs, even indexing them. You'll find them in Moz's index because those URLs are completely discoverable on the web. It appears to me Google is making a special case out of them. Although they don't do that for all Feedburner URLs.

From a larger picture point of view, there's always going to be a discrepancy between what Google counts and what you find in OSE, or Majestic, or Ahrefs.

So we don't really know how Google is treating those URLs. I would suspect they are using them for discovery. It's possible they pass link equity as well, but I wouldn't say that with a lot of confidence.

It's possible we'll considering treating these URLs different in the future if we become more confident in how we believe Google treats them.

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